Standardize CKM matrix

[scarlehoff]

The CKM matrix (should be) a list of numbers, however it is declared as a string

https://github.com/NNPDF/theories_slim/blob/04a3716991015717bb442038adbe25dec57d2650/data/theory_cards/700.yaml#L3

Personally I'd put a list everywhere. Is there a reason why a string would be preferred?

[felixhekhorn]

it was just historically a string since we were bound to sqlite ...

in yadism we wrote a conversion to support that https://github.com/NNPDF/yadism/blob/9db321d2f3496d8ba85ee9b7587b1b7231d8ecc3/src/yadism/coefficient_functions/coupling_constants.py#L460

[felixhekhorn]

do you want to do a list + .reshape(3,3) or directly a matrix?

[scarlehoff]

I see, if we have the conversion in separate programs we might as well convert it to a list everywhere.
Maybe directly a 3x3 matrix makes more sense? but that part I don't really mind